The present invention relates to a vehicle seat belt pretensioner and, in particular, to a pretensioner of the type that pretensions a seat belt by rotating the take-up shaft of a belt retractor.
In some vehicle seat belt systems, a pretensioner is provided for tightening the belt at the instant of a collision so that it firmly engages the occupant before the occupant is thrown forward. One type of seat belt pretensioner is disclosed in the Japanese Patent Laid-Open Publication No.1-119454.
In that pretensioner, a pulley is connected to a take-up shaft of a seat belt retractor through a coupling device, and the pulley is rotated by a cable, which is wound around it and is pulled by means of a drive unit (power source). The coupling device comprises a pair of gears, one of which is fixed on the retractor take-up shaft and the other of which is fixed on the pulley. The pulley is supported on a guide unit in the form of a pivoting arm. In the set position, the guide unit is maintained by a shear pin in a position in which the gears are not engaged. When the drive unit is operated, the shear pin is broken stepwise, first to allow the guide unit to pivot and then to allow the pulley to rotate. After the two gears are engaged with each other by swinging of the guide unit, the pulley is rotated and imparts rotation to the take-up shaft so as to wind a length of the belt onto the belt reel.
In a pretensioner of the type described above, a high power is applied instantaneously, in the order of milliseconds, and St is essential that the gears be engaged very rapidly in order to transmit rotation to the take-up shaft as soon as possible after the power source is activated. One significant cause of delay is a collision between the addenda of the gears when the driving gear moves toward engagement with the driven gear. Such a collision results in a sliding of the addendum of one gear along the addendum of the other gear and a cessation of movement of the driving gear into meshing engagement with the other gear, with a consequent significant loss of time and energy.